1. Field of the Invention
The present invention relates to a coaxial two-wheel vehicle and a method for controlling the same and, in particular, to a stabilization technique on the vehicle when an occupant is not thereon.
2. Description of Related Art
In recent years, movable bodies have been developed that detect their own attitude information using a gyro sensor, an acceleration sensor, etc., and that perform drive control based on the detected attitude information. In these movable bodies, by employing a principle in which attitude information of the body's own is detected from signals detected by the gyro sensor and the acceleration sensor to thereby control an attitude with an inverted pendulum, or a principle of ZMP (zero moment point) control used for controlling bipedal robots, a rotation command (specifically, a torque command, a speed command, and a position command) to a motor is calculated so as to keep its own attitude, and the resulting rotation command data is transmitted to a motor control unit. These movable bodies can keep their own attitudes with such feedback control, and can travel utilizing change of a center of gravity of the occupant.
For example, travel devices are proposed that travel with persons being thereon, and that have various vehicle body constitutions and vehicle structures, in which their own attitude information is detected and drive control is performed based on the detected attitude information. For example, in Japanese Unexamined Patent Application Publication No. 2006-211899 and Japanese Unexamined Patent Application Publication No. 2006-315666, a coaxial two-wheel vehicle in which two wheels are arranged coaxially is disclosed. We have now discovered that such the coaxial two-wheel vehicle has a characteristic that it is structurally unstable in a forward/rearward direction, so that wheels are controlled by a feedback from an attitude sensor to thereby stabilize a vehicle attitude. Additionally, operation of the vehicle, such as to travel forward, travel rearward, and turn right and left, is performed by an instruction using such as a movement of an occupant's center of gravity, inclination of a step, and a control handle. Alternatively, in some cases, remote control by a command input from an outside or an autonomous movement based on a trajectory planning of the vehicle's own may be performed.
In a general coaxial two-wheel vehicle, attitude control is not performed before a person gets on the vehicle and after he or she gets it off. Hence, the person needs to do some work, such as to support the vehicle with his or her hand(s) or to pull out and stand a retractable kickstand. Additionally, if the coaxial two-wheel vehicle is used on a slope, great force to support the vehicle or a kickstand with high strength may be needed. Further, if the occupant jumps off the vehicle, or he or she falls therefrom, there are such problems that only the vehicle spontaneously travels by a certain distance to thereby collide with others, or the vehicle is fallen to be damaged. Here, since positions of the center of gravity of a system including the occupant differ between at a time when the occupant is on the vehicle and at a time when he or she is not thereon, even if attitude control is performed when the occupant is not on the vehicle, attitude control cannot be performed well due to a same target pitch angle.
If the person wants to haul the vehicle before he or she gets it on or after he or she gets it off, as disclosed in Japanese Unexamined Patent Application Publication No. 2006-211899, it is required that haul control performed by a command input from some kind of input device under a state where the attitude control is stopped, or haul against friction of wheels, thus requiring time and effort.
On the contrary, it is a problem to stop the attitude control when the occupant is not on the vehicle. Since the coaxial two-wheel vehicle is structurally unstable, when attitude control is stopped, it cannot be self-standing and thus falls. Additionally, when the occupant gets off the vehicle during its traveling, it falls due to its inertia after traveling a certain distance. Since the target pitch angle is not changed according to a case where the occupant is on the vehicle and a case where he or she is not thereon, even if it is possible to perform attitude control only in either case, both cases cannot be dealt with.
The present invention is made to solve the above described problems, and an object thereof is to provide a coaxial two-wheel vehicle in which stability is enhanced when an occupant is not on the vehicle, and a method for controlling the coaxial two-wheel vehicle.